1. Technical Field of the Invention
The invention relates to a process for manufacturing metal parts with a sandwich structure. The metal parts are reinforced by fibers having a high elastic modulus and are joined together by diffusion bonding.
2. Description of the Related Art
Thin and lightweight parts made of metal alloys having a high strength/mass ratio, i.e. mainly aluminum, magnesium and titanium alloys, are used in aeronautics. In the case of turbomachines, these parts are typically casings, casing arms and blade assemblies. However, such alloys have the drawback of having a low Young""s modulus and the parts made with these alloys must consequently be strengthened by ribs so as to provide sufficient rigidity. The presence of these ribs however has the drawback of increasing the mass of the part. In addition, these ribs may have complex shapes and it may consequently become very expensive to produce them.
Composites consisting of reinforcing fibers embedded in a metal matrix are also used at the present time in aeronautics, the fibers possibly being made of silicon carbide (SiC), of boron or of carbon and the matrix made of aluminum, magnesium and titanium alloy. Such materials have mechanical properties that are substantially improved over the same matrix alloy used alone. By way of example, if a composite consisting of silicon carbide fibers embedded in a Ta6V titanium alloy matrix is compared with the Ta6V titanium alloy used alone, it is found that the mechanical strength is increased by 120%, the Young""s modulus is increased by 100% while the density is reduced by 15%.
Metal matrix composite are obtained essentially by strongly compressing, at the superplastic forming temperature of the metal alloy, a preform consisting of reinforcing fibers and a metal alloy, the fibers possibly being woven or wound and the metal alloy possibly being in the form of foils placed between the fibers, in the form of a coating applied around the fibers by the process called xe2x80x9cphysical vapor depositionxe2x80x9d or xe2x80x9cPVDxe2x80x9d, the metal alloy also possibly being applied by plasma spraying onto the woven or wound fibers. Hot pressing may be carried out in a die in a press when the shape of the part allows this pressing, that is to say when it has a predominantly plane shape. Otherwise, the pressing can also be carried out in an autoclave, the part then being surrounded by a container, that is to say a sealed metal shell in which a vacuum is created, the part also possibly being pressed against a former. Such processes allow thin composite parts to be produced which have improved mechanical properties compared with the same part made of metal alloy. However, the use of these processes for producing large parts throughout their thickness would require the use of a large quantity of fiber, whereas only the fibers at the surface of the part are contributing to the stiffness of this part according to a principle well known in the strength of materials. Thus, because the cost of purchasing and of using these high-strength fibers is very high, the manufacturing cost of such parts would be prohibitive.
Hybrid parts are also produced to comprise a fiber/metal alloy composite part and a part made of metal alloy alone. To manufacture such pieces, a blank of the second part is machined and the first and second parts are pressed together using the aforementioned general process, this hot pressing bonding the two parts together by mutual diffusion of the alloy of each part into the other part.
In general, the constructing and the hot pressing of a part made of a composite, comprising reinforcing fibers having a high elastic modulus embedded in a matrix made of a metal alloy, remain difficult operations since these fibers cannot withstand large curvatures without breaking, because of their high elastic modulus. Since the pressures required both for the densification and for the diffusion bonding are very high, in order for the fibers not to break, the following conditions are usually satisfied:
the fibers are arranged uniformly in plies, one beside another in a parallel manner;
during densification, the process must allow the matrix to flow very homogeneously around the fibers so as not to cause, due to the effect of the pressure, localized displacements of the fibers in which there would be a risk of breaking them.
By way of example, the pressing and diffusion bonding of a composite consisting of reinforcing fibers made of silicon carbide with a matrix made of Ta6V titanium alloy require a pressure of 600 to 800 bars at a temperature of about 900xc2x0 C.
The invention provides process for producing thin and rigid metal parts, said process comprising in particular the following operations:
production of a core made of a metal alloy;
application, to each face of the core, of a shell made of a composite which includes reinforcing fibers embedded in a metal alloy, said fibers having an elastic modulus at least equal to four times that of the metal alloy;
densification of the shells by pressing at least in the thickness direction at the superplasticity temperature of the metal alloy surrounding the fibers; and
diffusion bonding of the shells to the core by pressing at least in the thickness direction at the diffusion temperature of the metal alloys of the shells and of the core.
Such a process is noteworthy in that:
a plurality of emerging cavities is produced in the core on at least one face of the core, for example by drilling, by electrical discharge machining or by punching, with a lowest possible volume fraction of the core, for example 0.9, the volume fraction of the core being the ratio V/Vc in which Vc is the volume of the solid core and V is the volume of the remaining matrix of the core after the cavities have been produced, said cavities being uniformly distributed over the part; and
the shells are densified to the shape of the core before they are applied to the core.
This process has the effect of forming, in the part of the cavities closed at least on one side by the shells with negligible creep of said shells into said cavities, and has the result of simultaneously lightening and stiffening the parts, without increasing their thickness.
The curvature of the reinforcing fibers is kept approximately constant above each cavity and in the vicinity of each cavity, thereby simultaneously preventing the fibers from breaking or allowing only a negligible proportion of them to break, and permitting these fibers to be maintained in the best position so as to strengthen and stiffen the part.
Thus, contrary to what may have been thought, it is possible to press, under the aforementioned conditions and without appreciable creep, into the cavities of the composite shells which consist of reinforcing fibers having a high elastic modulus embedded in a matrix made of a metal alloy, onto a core which itself comprises a multitude of cavities open at its surface, the prior densification of the shells making said shells stiff enough to limit their creep into the cavities to negligible values.
Advantageously, the metal alloys will be taken from the group comprising titanium, aluminum and magnesium, and the reinforcing fibers from the group comprising silicon carbide, boron and carbon, so as to combine a light metal alloy with reinforcing fibers having a high strength and a high elastic modulus.
In a first embodiment of the invention, the diffusion bonding is carried out in a die in the press, for example with a heating die or with a furnace press. Such an arrangement has the effect of keeping the average thickness of material between the cavities at a sufficient value compatible with the pressing process employed and has the result of preventing the core from collapsing during the pressing in a die.
In a preferred method of implementing the process, the shells are bonded to the core by isostatic pressing in an autoclave. The width of the cavities must then be limited to a value compatible with this type of pressing. The present process can then be applied to parts which are impossible to produce in a die, for example turbomachine casings. It will be understood that the pressure applied to the part by a fluid, in this case the gas of the autoclave, favors creep of the shells into the cavities. However, it has been found that this creep may be regarded as being negligible when the dimensions of the cavities remain less than a certain limit which depends on the properties of the composite shell, thereby allowing the manufacture of parts under these conditions of aeronautical or aerospace quality.
Advantageously, a minimum volume fraction of the core will be used so as to reduce the mass of the core and to lighten the parts of the same stiffness and strength.
Also advantageously, the volume fraction of the core V/Vc will be increased in the vicinity of the regions for fastening the part. This has the effect of increasing the compressive strength of the part at these points and has the result of allowing the part to be bolted with high tightening torques. In one particular embodiment, the core will be solid in the immediate vicinity of said fastening members.
Advantageously, cavities touching each other may be machined in the core along suitable lines. This has the effect of forming ducts between the shells and has the result of allowing fluid to flow into the thickness of the part. This result is particularly beneficial in the case of structural parts of a turbomachine, such as the casings and the casing arms: it is thus possible to distribute lubricant, fuel or gas at various temperatures, especially in order to control the operating clearances.
Advantageously, these cavities touching one another each emerge only on one side of the core so as to maintain the cohesion of said core during production of the part.
Advantageously, when the part is a turbomachine blade assembly comprising a blade and a root at one end, the core extending into the blade and into the root, cavities with a reduced volume fraction of the core V/Vc will be made in the core of the blade and, optionally, cavities with a high volume fraction of the core V/Vc will be made in the root, thereby making it possible to produce very lightweight blade assemblies which will be able, however, to withstand high root embedment stresses. Advantageously, the blade assembly will be produced in a die, thereby allowing small volume fractions of the core and therefore a considerable weight saving.